The problems associated with the lubrication of automatic and manual transmissions and the operation of hydraulic fluid systems are well known to those skilled in the art. For example, in the lubrication of transmissions, proper fluid viscosity at both low and high temperatures is essential to successful operation. Good low temperature fluidity eases cold weather starting and insures that the hydraulic control system will properly "shift gears". Sufficient viscosity at elevated temperatures insures thin film lubrication, pumpability and the satisfactory functioning of converters, valves, clutches, gears and bearings.
In the operation of hydraulic fluid systems, proper fluid viscosity at both low and high temperatures is essential to successful operation. High temperature viscosity retention is beneficial in lubrication, contributes to streamline flow and reduces leakage. Good low temperature fluidity provides rapid control action, less heating loss and lower pressure drop.
These conflicting fluidity requirements call for a product that exhibits the following characteristics:
(A) high temperature viscosity retention;
(B) low temperature fluidity; and
(C) shear stability.
In order to prepare lubricants having these characteristics, it has become common practice to add a variety of chemicals to the oil. For example, in order to meet the viscosity requirements, compositions have been added to the oils which are characterized by relatively small change in their viscosity with changing temperature, and they are commonly graded according to SAE standards according to the viscosities at low (e.g., 0.degree. F.) and at high temperatures (e.g., 210.degree. F.). As a result of the incorporation of such additives, the lubricating oils are often referred to as being "multi-graded". In terms of widely accepted concepts, such multi-graded lubricants have the desirable properties of being able to function at cold temperatures and to continue to function satisfactorily as they become heated during operation.
Although chemical compositions have been developed which improve the viscosity characteristics of lubricating oil, it is often desirable to further improve the low temperature characteristics by including compositions which function as fluidity modifiers at low temperatures. Fluidity modifiers are capable of lowering the viscosity of a lubricating oil at low temperatures generally by retarding the formation of undesirable microcrystalline wax substances.
In addition to the above improvements, it is desirable, if not necessary, that lubricating compositions especially designed for use as transmission fluids, hydraulic fluids, and gear fluids exhibit shear stability. Shear stability means that the lubricating oils will not degrade or lose their desirable viscosity characteristics as a result of the shearing forces encountered during their use. Lubricating oil compositions exhibiting desirable shear stability will be found to generally have the viscosity within 85-100% of their original viscosity after a number of hours, (e.g., 100 hours) of service. It has been recognized that many ordinary viscosity index improvers commonly added to crankcase lubricating oils, such as high molecular weight polyisobutene and polyacrylates do not possess the desired shear stability for use in improving the viscosity characteristics of transmission fluids and hydraulic fluids.
It has now been found that multi-grade lubricants exhibiting improved shear stability can be made by utilizing the compositions of the present invention. These lubricants are particularly useful as additives in transmission fluids and hydraulic fluids.
U.S. Pat. No. 3,702,300 issued to Coleman relates to lubricating compositions containing a nitrogen-containing mixed ester of a carboxy-containing interpolymer.
U.S. Pat. No. 3,933,761 issued to Coleman relates to a nitrogen-containing mixed ester of a carboxy-containing interpolymer and lubricants containing the same.
U.S. Pat. No. 3,956,149 issued to Coleman relates to a lubricant or fuel composition containing a nitrogen-containing ester of a carboxy-containing interpolymer.
U.S. Pat. No. 3,959,159 issued to Coleman relates to lubricating compositions containing a nitrogen-containing mixed ester of a carboxy-containing interpolymer.
U.S. Pat. No. 4,284,414 issued to Bryant relates to a crude oil composition containing mixed alkyl esters of a carboxy-containing interpolymer.
U.S. Pat. No. 4,604,221 issued to Bryant et al relates to a Nitrogen-containing ester of a carboxy-containing interpolymer and lubricants containing the same.
U.S. Pat. No. 4,180,637 issued to Evani relates to a process for preparing a low molecular weight carboxy-containing copolymer.
U.S. Pat. No. 4,200,720 issued to Evani relates to a process for preparing a low molecular weight carboxy-containing interpolymer.